1. Field of the Invention
The present invention concerns scintillation spectrometers. More particularly the present invention is directed to scintillation counters comprising an inorganic scintillation crystal adapted to measure both beta and gamma emissions of radioactive samples.
2. Description of the Prior Art
There are two major types of scintillation equipment in use today. The first is what is commonly classified as a well-type gamma scintillation spectrometer and the second is the liquid scintillation spectrometer for counting beta emitting samples.
When ionizing radiation is absorbed by a suitable scintillator photons are emitted. These light photons can be detected by a photomultiplier tube which is a sensitive device for detecting a light pulse and coverting it into an electrical pulse.
The type of scintillator used depends on whether beta or gamma radiation is to be detected. For gamma radiation detection sodium iodide crystals activated with thallium are usually used. The presence of the relatively high atomic iodine ensures good absorption of gamma radiation in the crystal, which is also transparent to the fluorescent light emitted. Sodium iodide is hygroscopic so that it needs to be protected from the atmosphere. The crystal is usually encased in a thin aluminium alloy can, with an intermediate layer of a diffuse reflector to improve the efficiency of light collection. The base of the scintillation crystal which is not covered with reflector, is coupled to a photomultiplier tube.
Usually a well-type crystal is used. This is a cylindrical crystal with a hole cut into one end, the inner surfaces of which are covered with reflector and aluminium. The sample in a test tube is inserted into this hole. The geometry is very favourable because nearly all the gamma rays emitted by the sample will traverse the crystal.
Most well counters presently in use employ sodium iodide cyrstals 13/4 inch in diameter and 2 inches in height with a well 11/2 inches deep and three fourths inches in diameter, the inside diameter of the aluminum well liner being 0.656 inch or 16.5 mm. This well counter is often referred to as the "standard" sodium iodide well counter, however, 2.times.2 inch and 3.times.3 inch sodium iodide crystals are also very commonly used.
Whereas gamma radiation has high penetration power, beta radiation and especially low energy beta emitters like tritium and C-14, are easily absorbed within the sample itself. To avoid the problems of self absorption liquid scintillation spectrometers are used.
In the use of liquid scintillation spectrometers the sample is dissolved in liquid scintillator so that the radioactive molecules are in intimate contact with the scintillator. The light pulses resulting from the absorption by the liquid scintillator of the beta particles are then detected with a photomultiplier tube, which is placed in proximity to the sample vial itself.
Heretofore, for a mixture of radioisotopes which could not be counted adequately by gamma ray spectroscopy, beta as well as gamma counting was done in two separate counters. Thus the mixture would be counted in a standard sodium iodide well counter and then because of the inadequacy of said counter for counting beta rays the sample would then be placed in a beta ray counter.